1. Field of the Invention
The function of laminating adhesives is to join together very fine films of different or identical materials, e.g., polyethylene, polypropylene, polyester, polyamide, aluminum, paper or cardboard to form composite films used for many different purposes, e.g., for packaging or for decorative purposes. A suitable laminating adhesive is expected to provide excellent bonds on numerous substrates with only a small application of adhesive.
Further, a suitable laminating adhesive must be economically processible, i.e., it must be suitable for application in conventional processing machines (e.g., BILLHOFER under the usual operating conditions, illustrative of which are low drying temperatures and high conveyor belt speeds. According to the present state of the art, suitability is attained by means of solvent-containing one-component or multi-component systems. The organic solvents released in the course of processing constitute a problem to the adhesives processor, as he must use expensive suction devices for removal of the solvents and plants for the recovery or burning of the solvents.
It was therefore an object of the present invention to provide solvent-free aqueous polyurethane dispersions as laminating adhesives which would not have the above mentioned disadvantages of laminating adhesives containing solvents.
2. Brief Description of the Prior Art
Numerous processes for the preparation of polyurethanes containing carboxylate groups are known. Thus, for example, conventional prepolymers containing isocyanate end groups may be reacted with aqueous solutions of amino carboxylic acids or their salts in an organic solvent to form the corresponding polyurethane ureas containing carboxylate groups (see e.g., DE-AS 1 495 745, GB-PS 1 076 688, U.S. Pat. No. 3,539,483). The disadvantage of this process is that the polymer can only be synthesized in the presence of organic solvents which remain in the end product or must be removed from the end product by distillation, which entails an increase in the manufacturing costs.
According to another process, dimethylpropionic acid may be used as chain lengthening agent for synthesizing polyurethanes so that the free carboxyl groups are, to a large extent preserved, and the product may then be neutralized (see e.g., U.S. Pat. No.3,412,054, DE-OS 1 913 271).
Although the incorporation of dimethylpropionic acid in isocyanate prepolymers can be carried out without the use of solvents, the problem in many cases arises from how to dissolve the dimethylpropionic acid in the prepolymer at the required low reaction temperatures. Another disadvantage is that dimethylpropionic acid can only be incorporated by an isocyanate reaction, which results in relatively high viscosities of the resulting modified isocyanate prepolymers.
Although esters containing both hydroxyl groups and free carboxyl groups are known from the chemistry of dispersible ester resins (see e.g., DT-OS 2 323 546, U.S. Pat. No. 4,029,617, BE-PS 803 346 or U.S. Pat. No. 3,876,582), the reactive groups are randomly distributed in the resultant products in such a manner that structurally defined and in particular predominantly linear polyurethanes cannot be produced on the basis of these known products.
According to another known process, polyurethanes containing free primary or secondary amino groups are reacted with .beta.-propiolactone or the anhydride of a dicarboxylic acid so that a modification of the polyurethane with free carboxyl groups takes place (see DT-AS 1 237 306).
The use of polyethers or polyesters containing terminal OH groups and sulphonate or carboxylate groups in side chains for the preparation of anionic polyurethane dispersions has also been described (see e.g., DT-AS 1 570 615).
The use of diamines containing sulphonate or carboxylate groups has been considered for the preparation of polyesters containing anionic groups in side chains (see e.g., DT-AS 1 570 615).
The use of trimellitic acid derivatives for introducing carboxylic acid groups into polyurethanes has also been described (EP 0 000 171), see also O. Lorenz et al., Agnew. Makromol.Chem.63 (1977) 11-22. According to the described process, trimellitic acid anhydride is first esterified with a macroglycol in such a manner that only the anhydride ring is opened. 50 mol-% of the remaining carboxylic acid groups are then converted into alkali metal or tertiary ammonium salts and the remaining carboxylic acid and hydroxyl groups are converted into an isocyanate prepolymer by a reaction with polyisocyanates.
A disadvantage of this process is the high proportion of basic groups, tertiary amines or carboxylic acid anions in the presence of isocyanate groups. As known e.g., from E. Muller in Houben-Weyl, Methoden der organischen Chemie, 14/2, p 82, Georg Thieme Verlag, Stuttgart 1963 and J. H. Saunders and K. G. Frisch: Polyurethanes Chemistry and Technology I in High Polymers vol XVI, Wiley Interscience, New York, 1962, such basic groups catalyze the reaction of isocyanate groups with hydroxyl groups of alcohols and carboxylic acids to form urethane or acid amide groups but they also, at the same time, catalyze the trimerization reaction of isocyanates to form isocyanuric acid derivatives, which may lead to an uncontrolled increase in the functionality and hence gelling of the prepolymer. The use of such catalytically active components always entails an uncertainty factor in the reaction of polyisocyanates carried out on a technically relevant scale.